After a lot of study they found out the least likely wallet to be returned is the one with a lot of cash.
That is one item they did not include within the wallet. This is the last line of the article that was excluded becasue of the excerpt requirment:
The wallets were planted at random about a quarter of a mile apart. Snapshots were inserted into a clear plastic window inside the wallets, none of which contained money.
And you are most likely right. The percentages would have be much lower if money was inside the wallet.